准备
- 首先,
promise
有三种状态:pending
fulfilled
rejected
; promise
在实例化操作中, 有两个改变状态的方法,分别为resolve
,reject
;promise
有很多方法,详情请见mdn
, 本篇文章先实现promise
的核心api
:then
和catch
;
我们使用 es6
提供的 class
来实现
class MyPromise {
// 准备三个状态
static PENDING = 'pending';
static FULFILLED = 'fulfilled';
static REJECTED = 'rejected';
constructor(executor) {
this.status = MyPromise.PENDING; // 表示promise的状态
this.value = null; // 表示promise的值
try {
executor(this.resolve.bind(this), this.reject.bind(this))
} catch (error) {
this.reject(error)
}
}
resolve() {
}
reject() {
}
}
在这里 executor
就是传递过来的函数,可以接收 resolve
和reject
,这里将内部的两个方法给传入,如果在调用的过程中报错了会调用reject
方法
完善 resolve/reject
他们做的工作分为以下几部
- 将状态改为
pending
为fulfilled
或rejected
- 可以接受一个值为当前的
promise
的value
resolve(value) {
if (this.status === MyPromise.PENDING) {
this.status = MyPromise.FULFILLED;
this.value = value
}
}
reject(value) {
if (this.status === MyPromise.PENDING) {
this.status = MyPromise.REJECTED;
this.value = value
}
}
then
then
函数可以接受两个参数,分别为成功的回调函数和失败的回调函数,并且回调函数的默认为一个函数
- 状态为
fulfilled
执行第一个回调,rejected
执行第二个回调 - 回调函数中给传入当前的
value
then
的执行为异步的
then(onFulfilled, onRejected) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
if (typeof onFulfilled !== 'function') {
onRejected = value => value
}
if (this.status === MyPromise.FULFILLED) {
setTimeout(() => {
onFulfilled(this.value)
})
}
if (this.status === MyPromise.REJECTED) {
setTimeout(() => {
onRejected(this.value)
})
}
}
验证一下:
console.log(1)
new MyPromise((resolve, reject) => {
console.log(2)
resolve('成功')
}).then(res => console.log(res))
console.log(3)
// 打印 1 2 3 成功
当promise
里面有异步代码的时候,这个时候运行到.then
方法 状态为pending
,下来增加一下异步任务的处理
异步处理
当状态为pending
的时候,表示执行的是异步任务,这个时候我们可以增加一个callback
,把异步执行的内容添加到这个callback
中,当执行完异步代码的时候,会执行异步函数的callback
的任务
constructor(executor) {
// ...
this.callbacks = []; // 用来存储回调函数的容器
// ...
}
resolve(value) {
// ...
this.callbacks.forEach(({ onFulfilled }) => onFulfilled(value))
// 当执行到这里的时候 如果有onFulfilled 就说明已经执行完then方法给容器添加内容了。把resolve的值传递给onFulfilled
}
reject(value) {
// ...
this.callbacks.forEach(({ onRejected }) => onRejected(value))
// 当执行到这里的时候 如果有onRejected 就说明已经执行完then方法给容器添加内容了。把reject的值传递给onFulfilled
}
then(onFulfilled, onRejected) {
// ...
if (this.status === MyPromise.PENDING) {
this.callbacks.push({
onFulfilled: value => {
setTimeout(() => {
onFulfilled(value)
})
},
onRejected: value => {
setTimeout(() => {
onRejected(value)
})
}
})
}
}
验证一下:
new MyPromise((resolve, reject) => {
setTimeout(() => {
resolve('成功')
})
}).then(res => console.log(res))
// 打印 成功
then
函数可以链式调用,接下来我们完善一下
链式调用
链式调用的核心就是返回一个新的 promise
,当成功调用的时候调用新的promise
的resolve
,失败reject
,并且链式调用会把前一个的返回值当作下一个的 resolve
的状态
then(onFulfilled, onRejected) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
if (typeof onFulfilled !== 'function') {
onRejected = value => value
}
return new MyPromise((resolve, reject) => {
if (this.status === MyPromise.FULFILLED) {
setTimeout(() => {
const result = onFulfilled(this.value)
resolve(result)
})
}
if (this.status === MyPromise.REJECTED) {
setTimeout(() => {
const result = onRejected(this.value)
resolve(result)
})
}
if (this.status === MyPromise.PENDING) {
this.callbacks.push({
onFulfilled: value => {
setTimeout(() => {
const result = onFulfilled(value)
resolve(result)
})
},
onRejected: value => {
setTimeout(() => {
const result = onRejected(value)
resolve(result)
})
}
})
}
})
}
验证一下:
new MyPromise((resolve, reject) => {
setTimeout(() => {
reject('失败')
})
}).then(res => res, err => err).then(res => console.log(res))
// 打印 失败
如果.then
的回调函数返回的是promise
的情况也要做个处理
边界处理
实现前:
new MyPromise((resolve, reject) => {
setTimeout(() => {
resolve('成功')
})
}).then(
res => new MyPromise((resolve, reject) => {
resolve(res)
}),
err => err
).then(res => console.log(res))
// 打印 { "status": "fulfilled", "value": "成功", "callbacks": [] }
当判断返回值为 MyPromise
的时候,需要手动调用 .then
的方法取他的值,并且吧当前的 promise
的改变状态的函数透出给 then
方法
then(onFulfilled, onRejected) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
if (typeof onFulfilled !== 'function') {
onRejected = value => value
}
return new MyPromise((resolve, reject) => {
if (this.status === MyPromise.FULFILLED) {
setTimeout(() => {
const result = onFulfilled(this.value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
}
if (this.status === MyPromise.REJECTED) {
setTimeout(() => {
const result = onRejected(this.value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
}
if (this.status === MyPromise.PENDING) {
this.callbacks.push({
onFulfilled: value => {
setTimeout(() => {
const result = onFulfilled(value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
},
onRejected: value => {
setTimeout(() => {
const result = onRejected(value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
}
})
}
})
}
验证:
new MyPromise((resolve, reject) => {
setTimeout(() => {
resolve('成功')
})
}).then(
res => new MyPromise((resolve, reject) => {
resolve(res)
}),
err => err
).then(res => console.log(res))
// 打印 成功
到这里 .then
方法就实现差不多了,接下来实现 catch
方法
catch
catch 方法可以处理拒绝的状态和错误的状态:
catch(onFulfilled) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
return new MyPromise((resolve, reject) => {
if (this.status === MyPromise.REJECTED) {
setTimeout(() => {
const result = onFulfilled(this.value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
}
})
}
验证:
new MyPromise((resolve, reject) => {
reject('失败')
}).catch(res=> console.log(res))
// 打印 失败
道理其实和 then
是相同的,到这里主功能基本上就差不多了,但是有很多重复的地方,优化一下
优化后完整代码
class MyPromise {
// 准备三个状态
static PENDING = 'pending';
static FULFILLED = 'fulfilled';
static REJECTED = 'rejected';
constructor(executor) {
this.status = MyPromise.PENDING; // 表示promise的状态
this.value = null; // 表示promise的值
this.callbacks = [];
try {
executor(this.resolve.bind(this), this.reject.bind(this))
} catch (error) {
console.log(error)
this.reject(error)
}
}
resolve(value) {
if (this.status === MyPromise.PENDING) {
this.status = MyPromise.FULFILLED;
this.value = value
}
this.callbacks.forEach(({ onFulfilled }) => onFulfilled(value))
}
reject(value) {
if (this.status === MyPromise.PENDING) {
this.status = MyPromise.REJECTED;
this.value = value
}
this.callbacks.forEach(({ onRejected }) => onRejected(value))
}
parse({ callback, resolve, reject, value = this.value }) {
setTimeout(() => {
const result = callback(value)
if (result instanceof MyPromise) {
result.then(resolve, reject)
} else {
resolve(result)
}
})
}
then(onFulfilled, onRejected) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
if (typeof onFulfilled !== 'function') {
onRejected = value => value
}
return new MyPromise((resolve, reject) => {
if (this.status === MyPromise.FULFILLED) {
this.parse({ callback: onFulfilled, resolve, reject })
}
if (this.status === MyPromise.REJECTED) {
this.parse({ callback: onRejected, resolve, reject })
}
if (this.status === MyPromise.PENDING) {
this.callbacks.push({
onFulfilled: value => {
this.parse({ callback: onFulfilled, resolve, reject, value })
},
onRejected: value => {
this.parse({ callback: onRejected, resolve, reject, value })
}
})
}
})
}
catch(onFulfilled) {
if (typeof onFulfilled !== 'function') {
onFulfilled = value => value
}
return new MyPromise((resolve, reject) => {
if (this.status === MyPromise.REJECTED) {
this.parse({ callback: onFulfilled, resolve, reject })
}
})
}
}
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